There has been a great demand in the market for portable communication and computing devices such as a portable TV and cellular phone. All these devices need a small, light-weight and low-consumption display device, and development efforts have been made accordingly.
FIG. 6 shows a circuit diagram corresponding to a single pixel element of a conventional liquid crystal display device. A gate signal line 51 and a drain signal line 61 are placed on an insulating substrate (not shown in the figure) perpendicular to each other. A selection pixel element selection TFT 70 connected to the two signal lines 51, 61 is formed near the crossing of the two signal lines 51, 61. The source 70s of the selection pixel element selection TFT 70 is connected to a pixel element electrode 17 of the liquid crystal 21.
A storage capacitor element 85 holds the voltage of the pixel element electrode 17 during one field period. A terminal 86, which is one of the terminals of the storage capacitor element 85, is connected to the source 70s of the selection pixel element selection TFT 70, and the other terminal 87 is provided with a voltage common among all the pixel elements.
When a gate signal is applied to the gate signal line 51, the selection pixel element selection TFT 70 turns to an on-state. Accordingly, an analog image signal from the drain signal line 61 is applied to the pixel element electrode 17, and the liquid crystal 21 through the pixel element electrode 17, and the storage capacitor element 85 holds the voltage. The voltage of the image signal is applied to the liquid crystal 21 through the pixel element electrode 17, and the liquid crystal 21 aligns in response to the applied voltage for providing a liquid crystal display image. By disposing the pixel elements as a matrix as described above, the LCD is achieved.
The conventional LCD is capable of showing both moving images and still images. There is a need for the display to show both a moving image and a still image within a single display. One such example is to show a still image of a battery within area in a moving image of a cellular phone display to show the remaining amount of the battery power.
However, the configuration shown in FIG. 6 requires a continuous rewriting of each pixel element with the same image signal at each scanning in order to provide a still image. This is basically to show a still-like image in a moving image mode, and the scanning signal needs to activate the selection pixel element selection TFT 70 by the gate signal at each scanning.
Accordingly, it is necessary to operate a driver circuit which generates a drive signal for the gate signals and the image signals, and an external LSI which generates various signals for controlling the timing of the drive circuit, resulting in a consumption of a significant amount of electric power. This is a considerable drawback when such a configuration is used in a cellular phone device, which has only a limited power source. That is, the time a user can use the telephone under one battery charge is considerably short.
Japanese Laid-Open Patent Publication No. Hei 8-194205 discloses another configuration for display device suited for portable applications. This display device has a static memory for each of the pixel elements. FIG. 7 is a plan view showing the circuit diagram of the active matrix display device with a retaining circuit disclosed in Japanese laid-open patent publication Hei 8-194205. A plurality of gate signal lines 51 and reference lines 52 is disposed in a predetermined direction. And a plurality of drain lines 61 are disposed in the direction perpendicular to the predetermined direction. Between a retaining circuit 54 and a pixel element electrode 17, a TFT 53 is formed. By displaying image based on the data retained in the retaining circuit, the operation of a gate driver 50 and a drain driver 60 is stopped for the reduction of the electric power consumption.
FIG. 8 shows a circuit diagram corresponding to a single pixel element of the liquid crystal display device. On a substrate, the pixel element electrode is deposed in a matrix. configuration. Between the pixel element electrodes 17, the gate signal line 51 and the drain signal line 61 are placed perpendicular to each other. The reference line 52 is disposed parallel to the gate signal line 51, and the retaining circuit 54 is formed near the crossing of the gate signal line 51 and the drain signal line 61. A switching element 53 is formed between the retaining circuit 54 and the pixel element electrode 17. A static memory (Static Random Access Memory: SRAM), in which two inverters INV1 and INV2 are positively fed back to each other, works as the retaining circuit for holding the digital image signal. Since the SRAM dose not need to refresh the memory for retaining the data, the SRAM, which is different from DRAM, is suitable for the display device.
In this configuration, the switching element 53 controls the resistance between a reference line and a pixel element electrode 17 in response to the binary digital image signal held by the static memory and outputted from the retaining circuit in order to adjust the biasing of the liquid crystal 21. The common electrode, on the other hand, receives an AC signal Vcom. Ideally, this configuration does not need refreshing the memory when the image stays still for a period of time.
However, when the static RAM is used in the retaining circuit 54, the number of the required transistors of the retaining circuit is 4 or 6, resulting in the enlargement of the circuit. Also, if the static RAM is placed between the pixel element electrodes 17, the area for the pixel element electrode is reduced. Thus, the following problems result; the numerical aperture of the liquid crystal display device is reduced, and the display device can not be made compact because of the enlargement of the size of the pixel element.